For the semiconductor fabrication of semiconductor devices, metal layers and oxide layers are very commonly used. Most metal layers are formed through a physical deposition whereby these metal layers usually have tensile stress. Most oxide layers are formed through a chemical deposition whereby these oxide layers usually have compressive stress. Take a MEMS apparatus as an example. The residual stress of the MEMS apparatus is an equivalent stress combining the compressive stress with the tensile stress. The MEMS apparatus can integrate an application-specific integrated circuit (ASIC) and a MEMS together in the same surface, thereby simplifying its packaging process.
However, the MEMS structure of the MEMS apparatus will be affected by its residual stress. Take a common XY-axis accelerometer as an example of the MEMS apparatus. The tensile stress of the metal layer will curve the MEMS structure upward, and the compressive stress of the oxide layer will curve the MEMS structure downward. Since the oxide layer is formed with the bonding chemically produced, the oxide layer has a high temperature of deposition and the bond force may cause that the residual stress of the oxide layer is larger than the residual stress of the metal layer. Therefore, the residual stress of the oxide layer leads the MEMS structure to be curved downward. Even if the residual stress can be released through the rapid thermal annealing (RTA) system, the thermal expansion coefficient of composite material will also affect the MEMS structure. For example, the thermal expansion coefficient of aluminum is 23 ppm/° C., and the thermal expansion coefficient of the oxide layer is 0.5 ppm/° C. In this case, the thermal expansion coefficient of aluminum is 46 times the thermal expansion coefficient of the oxide layer. Since the temperature around the MEMS apparatus may change, the design of the MEMS apparatus should consider not only its residual stress but also the thermal expansion related with two different layered materials.
Generally, since MEMS apparatuses nowadays are easily affected by residual stress and temperature and do not have firm structures, these MEMS apparatuses are easily deformed. Moreover, if the structural strength of the MEMS apparatus is not strong enough, the MEMS apparatus may easily be curved due to the change of external force.